1. Field of the Invention
The present invention relates to an imaging optical system provided with an image-stabilizing (anti-shake) function.
2. Description of Related Art
In recent years, there has been a demand to miniaturize the imaging units incorporated in mobile electronic devices, such as mobile phones equipped with a camera and smart devices (smart phones, tablet computers, etc.) equipped with a camera. In order to miniaturize an imaging unit, it is known in the art to configure an imaging optical system using a bending optical system which reflects (bends) light rays using a reflector element such as a prism or a mirror. Using a bending optical system in an imaging unit makes it possible to achieve a reduction in thickness (slimming down) of the imaging unit, especially in the direction of travel of the incident light emanating from an object to be photographed.
In addition, there also has been a tendency for the demand to equip imaging optical systems that use such a bending optical system with a so-called image-stabilizing (image shake correction/shake reduction) system that is designed to reduce image shake on the imaging plane that is caused by vibrations such as hand shake, etc.
The applicant of the present invention has proposed an imaging optical system in Japanese Unexamined Patent Publication No. 2013-238848 which suppresses image shake by driving (moving) a front lens element(s) that is positioned on the object side of a reflector element along a plane orthogonal to the optical axis of the front lens element(s).
Furthermore, Japanese Unexamined Patent Publication No. H09-251127 discloses an imaging optical system, in which image-stabilizing is carried out by swinging a lens element within the imaging optical system about a predetermined swing center.
However, according to the diligent research carried out by the present inventor, it was found that the imaging optical system disclosed in the above-mentioned Japanese Unexamined Patent Publication No. 2013-238848 has a tendency to deteriorate in image quality due to coma occurring when the front lens element(s) is driven (moved) along a plane orthogonal to the optical axis of the front lens element(s); and when the driving amount (movement amount) of the front lens element(s) is increased in accordance with an increase in an image-stabilizing movement amount (image-shake correction amount), the deterioration in the image quality due to coma occurring becomes more prominent. Accordingly, the driving amount (movement amount) of the front lens element(s) is restricted due to the need to balance between the driving amount of the front lens element(s) and the suppression of aberration fluctuations, thereby causing difficulty in increasing the image-stabilizing movement amount.
Whereas, in the imaging optical system of the above-mentioned Japanese Unexamined Patent Publication No. H9-251127, deterioration of image quality due to occurrence of coma can be reduced compared to the imaging optical system of the above-mentioned Japanese Unexamined Patent Publication No. 2013-238848. However, color shift occurs in the image due to the swinging movement of the lens element, thereby causing the resolving power (MTF: Modulation Transfer Function) of the image to deteriorate. This deterioration in the resolving power due to color shift becomes especially troublesome when the focal length of the imaging optical system is long (e.g., at the long focal length extremity).